However, he cannot eat eggs or flour. Is there a substitute I can use to replace those ingredients without compromising the structure or flavor of the cake? Please provide substitutes that work for you- I don’t want to take any chances!
In addition, he cannot eat potato. I’ve seen substitutes that include potato starch, but I’d like to avoid that, if possible.

Best answer:

Answer by SueCan your dad eat egg substitutes? here is a link which tells about what to use for egg substitutes. As far as the flour goes, check your grocery or health food store for gluten free flours. I usually use rice flour as a wheat flour substitute but I have not tried it on a cake. Rice flour tends to absorb more liquid than wheat flour so the cake may be a little dry.